1. Field of the Invention
The invention relates generally to electromagnetic well logging instruments. More specifically, the invention relates to antennas for electromagnetic well logging instruments.
2. Background Art
Various well logging techniques are known in the field of oil and gas exploration and production. These techniques typically employ logging tools or “sondes” equipped with sources adapted to emit energy into subsurface formations from a borehole traversing the subsurface formations. The emitted energy interacts with the surrounding formations to produce signals that are then detected by one or more sensors on the logging tools. By processing the detected signals, a profile of the formation properties may be obtained.
Electrical conductivity (or its inverse, resistivity) is an important property of subsurface formations in geological surveys and prospecting for oil, gas, and water because many minerals, and more particularly hydrocarbons, are less conductive than common sedimentary rocks. Thus, measurement of formation conductivity (or resistivity) provides a useful guide to the presence and amount of oil, gas, or water.
Formation resistivity properties are typically measured with electromagnetic (EM) induction or propagation logging tools. These tools are generally referred to as EM logging tools in this description, regardless of whether it is an induction tool or a propagation tool. EM logging methods are based on the principle that time-varying electric currents in a coil (or antenna) produce time-varying magnetic fields, which then induce electric currents (eddy currents) in the conductive surroundings. The eddy currents in turn induce secondary magnetic fields that can induce voltages in detector coils.
Conventional EM logging instruments/tools typically use one or more longitudinally-spaced transmitter antennas operating at one or more frequencies to induce eddy currents at different depth of investigation (i.e., different distances into the formation from the borehole). These tools typically also include a plurality of receiver antennas that are spaced apart from the transmitter antennas along the tool axes. As noted above, the receiver antennas detect the secondary magnetic fields that are induced by the eddy currents in the formation. The magnitudes of the induced signals at the receiver antennas vary with the formation conductivities or resistivities. The signals detected at the receiver antenna are typically expressed as a complex number (phasor voltage). Formation resistivities can then be derived from the phase shift (φ) and amplitude difference (A) as measured by different receiver antennas disposed at different distances from the transmitter antenna.
The magnetic moment m of a coil or antenna may be represented as a vector, oriented in a direction parallel to the induced magnetic dipole. The magnetic moment has a magnitude proportional to the magnetic flux, which is a function of the area of the coil, the number of turns of the coil, and the amplitude of the current passing through the coil. Conventional EM instruments have antennas consisting of coils mounted on the instruments with their magnetic dipoles parallel to the instrument's longitudinal axis. These instruments thus have longitudinal magnetic dipole (LMD) moments. The LMD induces eddy currents in loops lying in planes perpendicular to the tool or well axis.
When analyzing stratified earth formations, the responses of EM logging instruments are strongly influenced by the conductive layers parallel to the plane of the eddy currents. In contrast, nonconductive layers interleaved between the conductive layers do not contribute substantially to the detected signals. Therefore, the existence of the nonconductive layers are often masked by the conductive layers and become undetectable by conventional EM logging instruments. This poses a significant problem because the nonconductive layers often are rich in hydrocarbons and their identification is the object of logging operations.
Methods have been proposed to detect nonconductive layers located within conductive layers. For example, U.S. Pat. No. 5,781,436 issued to Forgang et al. describes a method using EM logging instruments with at least one coil having its magnetic dipole axis oriented away from the longitudinal axis of the tool. Such antennas are referred to as tilted or transverse magnetic dipole (TMD) antennas. Other EM logging tools equipped with TMD antennas may be found in U.S. Pat. Nos. 4,319,191, 5,508,616, 5,757,191, 5,781,436, 6,044,325, and 6,147,496. A TMD tool may induce and/or detect eddy currents that flow in loops on planes not perpendicular to the tool or well axis. If a nonconductive layer interrupts such loops (e.g., the nonconductive layer intercepts the eddy current loops at an angle), the detected signals will be significantly impacted, making it possible to detect the existence or location of the nonconductive layers.
Conventional EM logging tools are implemented with antennas that are operable as sources (transmitters) and/or detectors (receivers). One of ordinary skill in the art would appreciate that the transmitter and receiver coils (antennas) have the same characteristics and a coil or antenna may be used as a transmitter at one time and as a receiver at another. In these conventional EM logging tools, whether TMD or LMD tools, the antennas are typically mounted on the mandrel or support member and spaced apart from each other along the longitudinal axis of the tool. This configuration is necessary because constructing solenoid coils with their axes perpendicular to the tool axis (e.g., TMD) requires a considerable amount of space within the logging instrument.
FIG. 1 shows a conventional tri-axial EM logging tool having magnetic dipole antennas oriented in orthogonal directions. As shown in FIG. 1, the tri-axial EM logging tool has three transmitter antennas (Tx, Ty, and Tz) spaced apart along the axis of the tool and three receiver antennas (Rx, Ry, and Rz) disposed at a distance from the transmitter antennas. The receiver antennas are also spaced apart from each other along the tool axis. Because these antennas are all spaced apart along the tool axis, they would respond to different volumes in the formation. Consequently, conventional resistivity measurement data inherently include measurement errors.
To overcome this problem, it is desirable to have an EM logging tool having a plurality of antennas with their magnetic dipoles co-located at a common location such that they are responsive to the same volume in the formation. U.S. Pat. No. 3,808,520 issued to Runge discloses a triple coil antenna assembly, in which three antennas are arranged in orthogonal directions on a spherical support. However, this spherical support and antennas need to be accommodated inside the cavity of the mandrel. Because the interior space of a mandrel is very limited, the diameter of the spherical support is also limited. This in turn limits the area of the coils, hence the magnitudes of the achievable magnetic moments.
There remains a need for better techniques to implement EM logging tools having co-located antennas with different magnetic dipole orientations.